Location of conducting and/or magnetic bodies



June 1, 1954 s. WHITEHEAD ET AL LOCATION OF connucrrmc AND/OR MAGNETICBODIES 2 SheetsSheet 1 Original Filed Jan. 9, 1946 'q n N) rryk June1954 s. WHITEHEAD EIAL 2,580,226

LOCATION .OF CONDUCTING AND/OR MAGNETIC BODIES Original Filed Jan. 9,1946 2 Sheets-Sheet 2 Patented June 1, 1954 LOCATION OF CONDUCTINGAND/OR MAGNETIC BODIES Stanley Whitehead, Leatherhead, and BenjaminBoston, Cheltenham,

E. R. A. Patents Lim land, a company of G Original application January9, 1946,

England, assignors to ited, Leather-head, Engreat Britain Serial No.

639,986. Divided and this application June 16, 1951, Serial No. 232,014

Claims priority, application Great Britain January 24, 1945 6 Claims.

The present invention relates to the location of conducting and/ormagnetic bodies, and particularly by the use of electro-magnetic means.

It is often desired to find objects which are hidden in a greater orsmaller volume in space, often when they are buried or otherwisesituated under the surface of the earth. In such a search advantage isgenerally taken of some property by which the objects differ from theirsurroundings. When dealing with conducting and/or magnetic bodies, suchas ore bodies, pipes, cables, joint or junction boxes, tanks, cisternsand the like, the electro-magnetic method of location may be employed.The principle of this method is to produce an alternating magnetic fieldover the volume to be searched and to detect the distortion in the fielddue to the presence of the body to be located by reason of its magneticpermeability and/or the eddy currents induced in it.

The field may be produced only in the immediate vicinity of search orelse it may be generated over a larger volume.

In the former application a two-coil locator is generally used, whereone coil sets up a local magnetic field while the other serves as thesearch coil. This type can only be used in locating objects at a smalldepth below the surface of the ground, since the locator sensitivity isinversely proportional to the sixth power of the distance from theobject. tween adjacent operators may occur unless they are provided withequipment of separate and individual frequencies.

Where sensitivity to objects placed at a greater depth is desired, themagnetic field is generated over a larger volume. One easy way ofproducing a magnetic field over a large volume is by means of analternating current passing through a conductor. The return path of thecurrent may either be through the soil or through another con= ductor.In the latter case a loop is formed which may consist of several turns,and it may be placed either outside the area of search or it maysurround a part or the whole of the area.

The problem of location of metallic and/or magnetic objects would undersuch conditions be simplified if a uniform field could be produced, butthis is generally impracticable. For this reason the method hithertoemployed in prospecting has been to measure the magnetic field at anumber of points by means of one or two search coils. The results areplotted, and from irregularities in the field the presence of searchedobjects is deduced.

It is the object of the present invention to In addition interferencebes 2 provide search equipment to make such searches more direct and toenable them to be carried out expeditiously in spite of the use of anon-uniform magnetic field.

Other objects of the invention will be apparent from the followingdescription in conjunction with the accompanying diagrammatic drawingsin which Figure 1 shows, by way of example, one arrangement forproducing an alternating magnetic field in an area to be searched.

Figures 2, 3 and 4 illustrate a Figure 5 is a circuit and compensatorfor of Figures 2, 3 and 4.

Referring to Figure 1 an area it to be searched lies outside a loop ofcable I I. A single turn loop is shown but two or more turns may be usedif desired. By way of example the loop may measure 150 x 300 yards. Agenerator [2 is connected to feed to the loop an alternating current ofa suitable frequency, for example 500 cycles per second. The generatorl2 may be of 6 kilowatts. The current in the loop generates in the area10 an alternating magnetic field which is distorted in the neighbourhoodof conducting or magnetic bodies. It will be understood that it may ifdesired be arranged that the area to be searched is that within theloop.

The search unit shown in comprises two coils 5'6 and 51 ly, relativelyto one another, coil 56 has three terminals the coil 51 has twoterminals 6! and 62, the terminals 60 and 6! being connected together.The part of the coil 56 which is between the terminals 58 and 59 servesas a balancing coil the effect of which is adjustable by means of thewiper on a potentiometer 63 connected between the terminals 58 and 53.The number of turns in, and the dimensions of, the coils 56 and 5? areso arranged, according to well known formulae, that the coil 57 and thepart of the coil 56 between the termi-' nal Eli and a mid-point betweenthe terminals 58 and 59 have equal area-turns whereby equal and oppositeelectro-motive forces are induced in them when the unit is placed in auniform mag netic field.

The unit has two output terminals 4| and 42 for connection to theamplifier of Figure 5 (to be described later) and two further outputterminals 43 and 44 for connection to the compensator of Figure 5.Proceeding from the terminal ll to the terminal 42 through the circuitsshown in Figures 2 and 3 the terminal i! is connected search unit, anddiagram of an amplifier use with the arrangement Figures 2, 3 and 4mounted co-axialand coplanar. The 58, 59 and 60, and

to the wiper of a potentiometer 52. The lower end of the resistanceelement of the potentiometer- 52 is connected to the wiper of apotentiometer 1i. and the lower end of the resistance element of thepotentiometer H is connected to the wiper on the potentiometer 63. Fromthe wiper on the potentiometer 63 the circuit is through the lower partof the resistance element of the potentiometer 63, through that part ofthe coil 55 which is between the terminals 59 and 60, and through thecoil to the upper end of the resistance element of a potentiometer 53.The wiper of this potentiometer is connected to the upper end of theresistance element of a further potentiometer 69 whose wiper isconnected to the terminal 42.

Ignoring any potentials which may be developed across the resistanceelements of the potentiometers 52, H, 53 and 59, if the wiper on the potentiometer 63 is set to a position at which the potential between thatwiper and the terminal 66, is equal to the potential between theterminals BI and E2, when the unit is placed in a uniform magneticfield, the potential appearing between the terminals M and 42 is zero.

The output at the terminals 4i and 42 is balanced relatively to earth bymeans of two substantially equal capacitors 34 and 35, a resistor 35 anda potentiometer 24 connected as shown in the drawing. Adjustment of thewiper on the potentiometer 2 3 also facilitates adjustment of the searchunit to provide zero output when the unit is placed in a uniformmagnetic field.

Rotation of the search unit about the axis thereof normal to the planeof the coils, that is to say normal to the plane of the paper in Figure2, may give rise to the appearance of output volt age. This voltage maybe reduced considerably by the provision of a number of coils, say four,as shown at 64, 55, 66 and 6"! mounted in the same plane as the coils 56and 51 but arranged eccentrically with respect to the axis of the coils56 and 5?. The eccentric coils E4 and 65 are connected in series anddeliver their voltages through a reversing switch 58 to the resistanceelement of the potentiometer B9. The eccentric coils 65 and 67 areconnected in series and deliver their voltages through a reversingswitch to the resistance element of the potentiometer H.

Two compensating coils l8 and I9 are also provided with their axesmutually perpendicular and perpendicular to the axis of the concentriccoils 56 and 51. These compensating coils l8 and I9 enable a balance tobe maintained between the outputs of the coils 56 and 51 during forwardor lateral rotation of the search unit about horizontal axes. The coilis is connected through a reversing switch 54 to the resistance elementof the potentiometer 52 and the coil I9 is connected through a reversingswitch 55 to the resistance element of the potentiometer 53.

In adjusting the unit the reversing switches 54 and 55 andpotentiometers 52 and 53 are adjusted until the desired balance isobtained when the unit is given limited rotary movements abouthorizontal axes. Likewise the reversing switches 68 and Hi andpotentiometers 69 and 1| are adjusted until the desired balance isobtained when the unit is rotated about the axis of the coils 56 and 51.

Referring to Figure 4 a housing of insulating. non-magnetic material hasan upper part 20 and a cover plate 2| bolted thereto. The search coils55 and 51- are fixed to the under side of the part 20 by means of anysuitable adhesiv'e. Coils 64 and 65 are also shown and are likewisefixed in position by any suitable adhesive.

The potential difference between the terminals :iii and 44 isapproximately proportional to the mean magnetic field in the region ofthe search unit,

Figure 5 is a theoretical circuit diagram of a compensator and amplifiersuitable for use with the arrangement of Figures 2, 3 and 4. The voltageappearing between the terminals 4! and 42 of Figure 2' is applied to theamplifier and the voltage appearing between the terminals 43 and 44 isapplied to the compensator.

The amplifier comprises three variable-mu amplifying valves V1, V2 andV3 and has input terminals ll and 42. The compensator comprises avariable-mu amplifying valve V4 and has input terminals 43 and M. Theoutput of the valve V4 is rectified by a rectifier 45. A part of therectified output is tapped off from resistors 50 and El, smoothed in acircuit 56 and applied through a lead ll as a bias to suitable grids ofone or more of the valves of the amplifier 3i, in this example to thecontrol grids of the valves V2 and V3. The D. C. voltage developedacross the rectifier 55 is also fed back to the control grid of thevalve V4 through a smoothing circuit comprising a resistor 48 andcondenser 49.

The valve V4 is preferably chosen to have the characteristics as thevalves V2 and V3 of which it controls the amplification so that therelation between the control bias applied through lead ii and theamplification of the valves V2 and V3 is substantially identical withthe same relation in the compensator valve V4. By suitably adjusting theratio of the D. C. voltage fed back to the grid of the valve V4 to theD. C. voltage applied to the grids of the valves V2 and V3, theamplifier output can be made independent of the mean field strength.Although one form of compensator has been described by way of example,other known or suitable compensating means may be provided.

We claim:

l. Search equipment for use in the presence of an alternating magneticfield for the detection of bodies which distort such a field, saidequipment comprising two coils of equal area turns, means for supportingsaid coils in co-axial and co-planar relationship, an impedance elementhaving two outer terminals and an intermediate terminal between said twoouter terminals, one terminal or each of said coils being connected tosaid intermediate terminal and the other terminal of said coils beingconnected respectively to said outer terminals, and indicating means forindicating differences of potential between said outer terminals of saidimpedance element.

2. Search equipment for use'in the presence of an alternating magneticfield for the detection of bodies which distort such a field, saidequipment comprising two coils of equal area turns, means for supportingsaid coils in co-axial and coplanar relationship, an impedance elementhaving two outer terminals and an intermediate terminal between said twoouter terminals, one terminal of each of said coils being connected tosaid intermediate terminal and the other terminal of said coils beingconnected respectively to said outer terminals, a plurality ofcompensating coils connected to apply compensating electromotive forcesin series with said outer terminals, means for supporting said.-compensating coils in fixed relation to the said two coils, means foradjusting the said compensating electromotive forces, and indicatingmeans for indicating differ ences of potential between said outerterminals of said impedance element.

3. Search equipment for the location of electrically conducting bodiesand magnetic bodies located in an alternating magnetic field, comprisingtwo search coils each having a plurality of turns and the respectiveproducts of the mean areas enclosed by the turns of said coils and thenumbers of turns respectively in said coils being substantially equal, amechanical structure for fixedly supporting said coils one within theother in substantially coaxially and co-planar relationship, and meansfor indicating the difference between the electromotive forces generatedby said field in said two search coils.

t. Search equipment as claimed in claim 3, comprising two compensatingcoils mounted upon said mechanical structure with their axes mutuallyperpendicular and each perpendicular to the axes of said search coils,connections for applying voltage from said compensated coils to saidindicating means, and means for adjusting the electromotive forcesgenerated in said compensating coils to said indicating means for thepurpose of balancing.

5. Search equipment as claimed in claim 3, comprising an amplifier,means coupling said amplifier to said search coils, and means couplingsaid amplifier to said indicating means, said coupling means betweensaid search coils and said amplifier input comprising a balancingcircuit including an impedance element having an adjustable intermediatetap, two substantially equal capacitors connected in series between theouter terminals of said impedance element, said search coils beingconnected to the two outer terminals and the tap of said first impedanceelement, and the input terminals of said amplifier being connected tothe two outer impedance element.

6. Apparatus for the location of electrically conducting bodies andmagnetic bodies, comprising means including an electrically conductingloop and alternating current supply means connected to said loop forproducing an alternating magnetic field of substantial strength over alarge area, and search apparatus for searching over the area, the searchapparatus being movable relatively to said loop and including two coilsof equal area turns, means for supporting said coils in co-axial andco-planar relationship, an impedance element having two outer terminalsand an intermediate terminal between said two outer terminals, oneterminal of each of said coils being connected to said intermediateterminal and the other terminal of said coils being connectedrespectively to said outer terminals, a plurality of compensating coilsconnected to apply compensating electromotive forces in series with saidouter terminals, means for supporting said compensating coils in fixedrelation to the said two coils, means for adjusting the saidcompensating electromotive forces, and indicating means for indicatingdifierences of potential between said outer terminals of said impedanceelement.

terminals of said first References Cited in the file of this patentUNITED STATES PATENTS Number Name Date 1,812,392 Zuschlag s June 20,1931 2,124,577 Knerr July 26, 1938 2,337,352 Sitterson et a1. Dec. 21,19423 2,513,745 Reynolds July 4, 1958 2,557,994 Ostlund June 26, 1951

